The present invention relates to thyristor voltage switching circuits, and in particular to improved thyristor voltage switching circuits which are protected when in their off state against transient line voltages of an amplitude in excess of the breakdown voltages of the thyristors.
Solid-state relays for switching a.c. voltages are often used in applications such as traffic control equipment, furnace heater controls, motor controls and the like. Such relays are customarily four terminal devices (two input terminals and two output terminals), and use thyristor (SCR or triac) devices to switch an a.c. line voltage between the two output terminals in response to a signal across the two input terminals. The signal may be applied across the input terminals either by semiconductor logic or reed relays, and is ordinarily timed to switch the thyristor on when the a.c. line voltage is at its zero-crossing point to avoid a current surge therethrough. When the input signal is removed, the thyristor regains its blocking state and becomes nonconductive when the line voltage passes through its zero-cross point.
Such solid-state relays offers many advantages over their mechanical counterparts, the electromechanical relays. For example, with the solid-state relay there are no moving parts and no contact bounce. Further, the relay is capable of fast operation, is shock and vibration resistant, and does not emit audible noise when operated.
Unfortunately, there are certain disadvantages to such relays, the most troublesome of which is the inherent resistance of the thyristor, when in its off state, to line voltage transients. Line voltages normally encountered are not clean sine waves, and superimposed thereon are transient voltage spikes from motors, solenoids, electromechanical relays, lightning, switches, etc. A thyristor has a finite off state breakdown voltage rating, and must be protected from such transients. If not so protected, upon the occurrence of a transient voltage of an amplitude in excess of the breakdown voltage rating of the thyristor, the thyristor may be permanently damaged and thereafter fail to block voltage or turn off. This occurs, when the thyristor is off, as a result of a surge current passing through a point area of the thyristor junction upon a breakdown thereof.
Many solid-state relays use a triac as the voltage switching device. While a triac is capable of handling relatively large line currents when in its conductive state, it is readily destroyed when in its nonconductive state by transient line voltages of an amplitude in excess of its breakdown voltage. Conventional attempts to protect triacs against transient voltages include shunt-type circuit devices connected across the triacs between the anode and the cathode terminals thereof. Such shunt devices may include metal oxide varistors or zener diodes connected back-to-back and selected to conduct at a point when the voltage across the triac is above the nominal value of the switched voltage, but below the triac breakdown voltage. The devices are intended, when the triac is in its off state, to conduct and shunt surge current around the triac upon the occurrence of a voltage transient, to thereby protect the triac.
Such devices are useful in protecting triacs. However, the devices have an inherent resistance, and triacs are thus nevertheless often destroyed by the occurrence of line transients of an amplitude sufficient to generate across the devices, and therefore across the triacs, voltages in excess of the breakdown voltages of the triacs. Furthermore, as the devices are repeatedly stressed by the occurrence of large amplitude line transients, failure of the devices frequently occurs with attendant subsequent failure of the triacs.